linux-kernel - Re: PCI: Disable not requested resource types in pci_enable

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Message-ID: <eeae5c45-a718-71d8-04e8-4042eaed54de@gmail.com>
Date:   Wed, 15 Jul 2020 20:22:51 +0200
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PCI: Disable not requested resource types in pci_enable_resources

On 10.07.2020 23:09, Bjorn Helgaas wrote:
> On Thu, May 28, 2020 at 08:47:12PM +0200, Heiner Kallweit wrote:
>> Currently, if both resource types are enabled before the call, the mask
>> value doesn't matter. Means as of today I wouldn't be able to e.g.
>> disable PCI_COMMAND_IO. At least my interpretation is that mask defines
>> which resource types are enabled after the call. Therefore change the
>> behavior to disable not requested resource types.
>>
>> At least on my x86 devices this change doesn't have side effects.
> 
> Does this have a practical benefit?  If it fixes a bug or if there's
> something useful we can do because of this patch, I'll push it higher
> up the priority list.
> 
There's no big benefit. The current behavior just doesn't seem to be
consistent, and I don't see why we have a function pci_enable_device_mem().
Also after calling this function IO resources can be active.
So why not remove this function and use pci_enable_device() always.

Small benefit is that the change allows to guarantee that IO resources
are disabled after calling pci_enable_device_mem().
This might help to avoid using IO resources mistakenly in a driver.

>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
>> ---
>>  drivers/pci/setup-res.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/pci/setup-res.c b/drivers/pci/setup-res.c
>> index d21fa04fa..6ef458c10 100644
>> --- a/drivers/pci/setup-res.c
>> +++ b/drivers/pci/setup-res.c
>> @@ -459,8 +459,8 @@ int pci_enable_resources(struct pci_dev *dev, int mask)
>>  	int i;
>>  	struct resource *r;
>>  
>> -	pci_read_config_word(dev, PCI_COMMAND, &cmd);
>> -	old_cmd = cmd;
>> +	pci_read_config_word(dev, PCI_COMMAND, &old_cmd);
>> +	cmd = old_cmd & ~(PCI_COMMAND_IO | PCI_COMMAND_MEMORY);
>>  
>>  	for (i = 0; i < PCI_NUM_RESOURCES; i++) {
>>  		if (!(mask & (1 << i)))
>> -- 
>> 2.26.2
>>